1. Field of the Invention
This invention relates generally to the rapid thermal processing of objects heated by electromagnetic radiation within the visible, infrared and other near-visible wavelength ranges and, more specifically, to the efficient, controlled and uniform heating of an object in a rapid thermal processing reactor by uniform radiant heating of the object by a radiation source, whereby heat in the center of the object is influenced by a heat absorbing thermal insert located in close proximity to the underside of the object.
2. Description of the Prior Art
In the case of Ferroelectric material processing, rapid uniform heating is crucial in achieving useful remanent polarization characteristics. As for processing of dielectric materials, uniform heating is also critical in achieving low, medium and high dielectric constants for these materials. Equipment is needed to accomplish the required heat treating steps in a very rapid and efficient manner so that the total time at high temperature is minimized, so that throughput is maximized, and so that heating of the substrate is accomplished in a uniform manner. As the lateral feature sizes of microelectronic devices continue to shrink, scaling laws require that vertical dimensions be reduced correspondingly. In particular, vertical dimensions or junctions defined by impurity diffusions must be made more and more shallow. However, these shallow dimensions or junctions will diffuse to greater depths in the device substrate during the various heat treating steps required in the device fabrication.
To meet this requirement, so called Rapid Thermal Processing (RTP) tools (reactors) have been developed and widely used in the fabrication of silicon-based and gallium arsenide-based integrated circuits and in the production of large flat panel displays. To achieve the fast thermal response desired, RTP tools or reactors commonly process one substrate at a time. In silicon processing, the substrate is a silicon wafer and, for convenience, typical substrates may be referred to as xe2x80x9cwafers.xe2x80x9d This is not to limit the scope of this invention to only the processing of silicon devices; indeed, the invention includes, but is not limited to, the processing of gallium arsenide devices, circuits built on silicon-on-insulator (SOI) substrates, flat panel displays, magnetic storage media, and magnetic disk head devices. An additional advantage of single wafer processing tools is that production facilities using such tools can be optimally configured for maximum throughput and minimum cycle time.
Currently, RTP tools heat wafers by direct thermal radiation. That is, a high temperature heat source, such as tungsten-halogen lamps or argon arc lamps, heats the wafer by direct radiation. Unfortunately, there are several disadvantages associated with such direct radiation.
When heated in a reflective enclosed volume, such as an enclosed process chamber, the wafer exhibits spatial temperature nonuniformities. Such nonuniformities vary through the different temperature ramping and steady state steps of the RTP cycle and are dependent upon the relative temperatures of the lamps, the wafer and the walls of the enclosed process chamber. Segmenting the heat source into spatial zones and controlling such zones based on local temperature measurements has been only marginally successful in controlling these nonuniformities. Moreover, such temperature zone control requires complex and expensive hardware and software.
More commonly, a guard ring is provided to surround the wafer. The guard ring allows most of the temperature nonuniformities to occur within the guard ring rather than within the wafer. Although this approach is more economical, it is only a limited solution.
The instant invention is directed to a method and apparatus for uniformly heating an object in an enclosed volume. This invention provides for the enclosed volume or process chamber, an object such as a semiconductor wafer disposed within the enclosed volume, a heat source such as, but not limited to, tungsten-halogen lamps, the heat of which is directed to the enclosed volume through a transparent window, a guard ring, and a configured thermal insert of stainless steel or aluminum metal or some other chamber compatible material underlying the semiconductor wafer or other object. Preferably, the object is a semiconductor wafer processed in the enclosed process chamber of the rapid thermal processing reactor. The transparent window allows radiant energy to transmit to the semiconductor wafer. The semiconductor wafer, if not for the presence of a geometrically configured thermal insert, would receive and absorb thermal energy through its upper surface to be distributed vertically through the structure of the semiconductor wafer, that thermal energy being distributed (e.g., at steady state) and being of higher concentration at the central regions (laterally) of the semiconductor wafer than that thermal energy found at the outer regions (laterally) near or adjacent the edges of the semiconductor wafer. This is due to the larger radiating surface area at the edges making them cooler (at steady state). Introduction of the geometrically configured thermal insert, generally referred to as the thermal insert, influences the temperature of the semiconductor wafer by heat absorption. The thermal insert exhibits a low profile dome-like structure, having a specific spherical radius or diameter or other suitable geometric design or shape, which absorbs or draws off heat, at appropriate and different rates, from the semiconductor wafer to provide uniform temperature across the breadth of the semiconductor wafer during and after the heating process. The thermal insert is located in close proximity to the undersurface of the semiconductor wafer such that the vertical distance from the thermal insert and the semiconductor wafer gradually increases when measured outwardly in increasing increments from the centers of the thermal insert and the semiconductor wafer. Heat is extracted, absorbed or transferred away from the center of the semiconductor wafer by the underlying thermal insert at a rate higher at the center than at or near the edges to compensate for the characteristic temperature rise. The rate of absorption decreases from the center region toward the outer region depending upon the spherical radius or other geometrically configured upper surface of the thermal insert.
In one embodiment, the transparent window material and the enclosed process chamber is hexagonal, circular or of other suitable shape, and the thermal insert is positioned extremely close to the semiconductor wafer and to a concentric guard ring which minimizes lamp radiation from reflecting off the surfaces of the wager and the enclosed process chamber and reaching the backside of the wafer where non-contact temperature measurement of the wafer would have been made. In this preferred embodiment, temperature measurement errors induced by lamp radiation are minimized by the concentrically located guard ring.
According to one embodiment of the present invention, there is provided an apparatus for direct radiant heating in a rapid thermal processing reactor, the major components of which include a reactor body, an enclosed process chamber central to the reactor body, an inlet manifold and an outlet manifold opposingly located and attached to the reactor body for porting to the enclosed process chamber, an access port, quartz or ceramic, wafer or guard ring, support pins, a pyrometer, a thermal insert residing in the enclosed process chamber and aligned over the wafer support pins and/or a thermocouple support pin to the bottom of the reactor body, a guard ring concentrically aligned with the thermal insert as well as with the pyrometer, a transparent window aligned and secured to the upper region or top of the reactor body, a window cooling ring, a lamp box enclosure including a top where the lamp box enclosure secures to the upper region or top of the reactor body, tungsten-halogen lamps or other heat sources, and a plurality of water cooling tubes in and about the reactor body, the lamp box enclosure and lamp box enclosure top, and the window cooling ring.
One significant aspect and feature of the present invention is the use of a heat absorbing insert for transferring of heat from the center region of a semiconductor wafer to provide uniform heating along and about the breadth of a semiconductor wafer.
Another significant aspect and feature of the present invention is a heat absorbing thermal insert exhibiting a dome-shaped top surface.
Yet another significant aspect and feature of the present invention is a thermal insert which locates in close proximity to the underside of a semiconductor wafer for heat absorption therefrom.
A further significant aspect and feature of the present invention is the use of a guard ring to minimize radiant energy from reaching the backside of the wafer, and hence, the pyrometer apparatus.
Having thus described embodiments and significant aspects and features of the present invention, it is the principal object of the present invention to provide a method and apparatus for uniform direct radiant heating in a rapid thermal processing reactor.